1. Field of the Invention
The present invention relates to a method for fusion splicing the optical fibers. More particularly, the present invention relates to a method for fusion splicing the optical fibers with one fiber coating of a large thickness.
2. Description of the Related Art
A related-art method for fusion splicing the optical fibers of the same kind is known. In such a fusion splicing method, after the end faces of the optical fibers are fused together by discharge heating and butt jointed together, the optical fibers are subjected to additional discharge heating and pull-back (e.g., refer to Japanese Patent Unexamined Publication No. Hei. 7-248426, Japanese Patent Unexamined Publication No. Sho. 61-143704, etc.). FIGS. 5A-5D are views schematically showing the method for fusion splicing the optical fibers. Reference numeral 1 denotes a glass fiber, 2 denotes a core portion of optical fiber, and 3 denotes a micro bend.
FIG. 5A is a view showing a state where the core portions 2 of a pair of optical fibers 1 to be spliced are matched and aligned with a predetermined end face spacing. In this state, the discharge heating is performed to fuse the end portions of optical fibers, and then the ends of optical fibers are butt jointed together, as shown in FIG. 5B. When the core portions 2 are aligned and fusion spliced, the outer diameters of optical fibers are not matched if the core portions are eccentric, whereby the optical fibers are fused together in an offset state. Usually, the additional discharge heating is performed to modify this offset. By this additional discharge heating, the self-alignment action occurs owing to a surface tension of glass, so that the outer diameter portions of optical fibers are modified to be smoothly continuous. It has been found that the splice loss is improved by this additional discharge heating.
FIG. 5C is a view showing a state of optical fibers after the additional discharge heating. As a result of the outer diameters of optical fibers modified at the fused portion, a micro bend 3 is produced in the core portion 2. This micro bend 3 increases the splice loss, whereby the optical fibers butt jointed together at the time of fusion splicing are pulled back. FIG. 5D is a view showing a state after pulling back the optical fibers, in which the microbend 3 in the core portion is stretched into approximately linear form. Thereby, the splice loss can be further improved.
In fusion splicing the submarine optical fiber cables, to increase the splice strength of optical fiber, an fiber coating portion of optical fiber at the time of fusion splicing is clamped on a V-groove board (e.g., refer to Japanese Patent Unexamined Publication No. Hei. 6-118251). Further, to reduce a misalignment of the core portions of the optical fibers that is likely to occur by cantilever support, it is required to make the length (cleave length) of a glass fiber portion exposed in the fusion spliced portion as short as possible.
FIG. 6 is a view showing the clamp method. In the same figure, reference numeral 1 denotes a coated optical fiber, 4 denotes a glass fiber, 5 denotes a fiber coating, 6 denotes an axial clamp, and 7 denotes a V-groove board clamp. The axial clamp 6 clamps the coated optical fiber 1 and drives it in the axial direction to adjust the end face distance, and butt and pull back the optical fibers. The V-groove board clamp 7 is in a free state relative to the axial movement of the coated optical fiber 1, and holds the top end portion of the fiber coating 5 that is located near the glass fiber 4 having the coating removed. The V-groove board clamps 7 drive one glass fiber 4 of one coated optical fiber 1 in the X-axis direction, and the other glass fiber 4 of the other coated optical fiber 1 in the Y-axis direction to make alignment of the core portions of the optical fibers.
The fusion splicing method of FIGS. 5A-5D and the clamp method of FIG. 6 as shown in the related art are the case where the optical fiber outer diameters, the core portion diameters and the fiber coating diameters are the same or substantially same. In this case, the splice loss in fusion splicing has typically a predetermined correlation between the estimated loss and the actual measured loss. The estimated loss can be obtained by measuring the misalignment of the core portion in the splicing portion and the inclination of the core portion in the splicing portion, and the splice loss is calculated from this estimated loss.
However, when the optical fibers are fusion spliced, at least one of optical fibers having a fiber coating of large diameter, by the clamp method of FIG. 6, it has been found that there is no correlation between the estimated loss and the actual measured loss. For example, there is a case where an optical fiber having and a glass fiber diameter of 125 xcexcm and a fiber coating outer diameter of 0.4 mm in which the effective core cross section is expanded for use in the wavelength division multiplexing transmission with the submarine optical fiber cable, is fusion spliced with a dispersion shift fiber for hermetic seal having a glass fiber diameter of 125 xcexcm, with the surface coated with polyimide, and a fiber coating outer diameter of 0.25 mm, wherein each optical fiber is cleaved to have the length of glass fiber of about 3 mm. In this case, comparing the estimated loss and the actual measured loss using the clamp method in the above fusion splicing method, there is no correlation and a fully random relationship.
This may be caused by a plurality of factors, including a difference in the outer diameter between the optical fiber coatings and a bending dependency of optical fiber, but its detailed reason is not clear. Also, the splice loss is reduced by performing the additional discharge heating and pull-back. However, it has been found that there is no full correlation between the estimated loss and the actual measured loss in the related-art estimation method. However, in a submarine optical fiber cable system, for example, the estimated loss is employed in the case where the splice loss is practically difficult to measure.
The present invention has been achieved in the light of the above-mentioned problems. It is an object of the invention to provide a method for fusion splicing the optical fibers with a low splice loss and a high strength in which the actual splice loss can be estimated from the estimated loss in the case where optical fibers with one fiber coating of a large diameter are fusion spliced in short cleave length.
According to the present invention, there is provided a method for fusion splicing the optical fibers, comprising; aligning cores of the optical fibers to be spliced; fusing ends of the optical fibers together with their respective cores in alignment to splice; and measuring an inclination angle of the fusion-spliced optical fibers with reference to the optical fibers with their respective cores in alignment before fusing to estimate a splice loss.